Binder-lubricants for ferrous powder metallurgy

ABSTRACT

A binder-lubricant for metallurgical powder compositions for powder metallurgy (P/M) applications having a melting point between 50 and 100° C. and comprising glyceryl esters of fatty acids. Addition of such binder-lubricant has improved resistance to dusting, and has improved green density for compacting temperature between 50 and 100° C.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 USC 119(e) of prior U.S.provisional application no. 60/641,770 file Jan. 7, 2005, the contentsof which are herein incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to a method of fabricating ferrous-basedpowder compacts having high green densities.

The production of powder metallurgy (P/M) components with high staticand dynamic properties for high performance is increasingly required bythe P/M industry. In particular, it is well known that increasing thesintered density of parts results in a significant improvement in staticand dynamic properties. The final sintered density and mechanicalstrength of P/M parts are not only dictated by the powder formulation,but also by the compaction process and compacting conditions used, thepart characteristics and the sintering behavior. The densification andejection performance of powder mixes remains however one of the mainfactors to address when targeting very high density.

In practice, lubricants are commonly admixed to the metallic powderformulations to reduce the friction between the powder particlesthemselves and with the die walls of the tooling. This is needed toimprove the compressibility of metal powders, the uniformity ofdensification throughout the part, and also lowers the ejection forcethat is required to remove the compact from the die, thus minimizing diewear. For cold compaction, which is the process predominantly used,conventional lubricants are metallic stearates or amide-based waxes. Theuse of these conventional lubricants does not generally yield high greendensity at room or moderate temperature (lower than 100° C.).

A green-state compact is an intermediate step in the powder metallurgymanufacturing process, which is produced when a metal powder-lubricantmixture is compacted in a press. This compact is subsequently sinteredin a furnace to produce the finished product. Different processes arebecoming increasingly available to the P/M industry to improve thedensification and ejection performance of metallic powder mixes. Thewarm pressing process, which consists in pressing a preheated powder mixin a heated die (most often between 100° C. and 180° C.), enables thefabrication of parts with high density and green strength by increasingthe ductility of the ferrous powder particles. The gain in densityachieved by warm compaction versus cold compaction generally rangesbetween 0.12 to 0.30 g/cm³. The density gain is usually larger forhigher compacting pressure and less compressible powders. The warmpressing process requires the use of specific presses and toolings. Inaddition, to take advantage of the beneficial effect of an increase ofthe compacting temperature on densification, powder mixes must beproperly designed, in particular the selection of the internal lubricantto provide adequate lubrication at die walls during both the compactionand ejection steps.

The die wall lubrication technique is also a promising avenue to promotegreen densities when high compacting pressures are used. This techniquehas been the object of several studies in recent years. The benefits ofthis technique consist in the possibility to significantly reduce theinternal lubricant level in the powder mix, while maintaining goodlubrication at die walls during the compaction and the ejection ofparts. Even though the die wall lubrication has been extensively studiedat the laboratory scale, it is not widely used on a production scalebecause of the difficulty in controlling the amount of lubricant andespecially the thickness and uniformity of the film deposited on diewalls, and also because of the risk that improper die wall lubricationoccurs sporadically causing accelerated die wear and even toolingseizure.

Glyceryl esters of fatty acids are known as admixed lubricants for thecold compaction of iron-based powders. Molera et al. in “Possiblealternative lubricants for processing iron powders”, Powder Metallurgy,1988, vol.31, n°4, p.281, evaluated specific triglyceryl esters of fattyacids and showed that these substances lead to notably inferior greenand sintered properties than conventional P/M lubricants and thereforedismissed its use. Meyer et al. in “Considerations on the practicaleffects of lubricants and binders commonly used in compacting metalpowders”, Powder Metallurgy, 1969, vol.12, n°24, p.298 carried out asystematic compactibility study on an iron-based powder admixed withseveral lubricants in proportions of 0.25, 0.5, 0.75, 1% of the totalweight. In particular, they refer in their article to the use of an“organic stearate” that corresponds to a specific glyceryl ester offatty acid. Ramstedt et al. in “Powder composition” Patent ApplicationPublication No: US 2003/0230166 A1, Dec. 18, 2003 filed a patentapplication claiming the use of glyceryl stearate as lubricant foriron-based powder. Their claim is based on the fact that according totheir prior art, these substances have been used as binders in thepowder metallurgy field but never admixed as lubricants. Hendrickson etal. in U.S. Pat. No. 6,602,315, Aug. 5, 2003 claim improvedsegregation-resistant and dust-resistant metallurgical compositionscomprising a coating material also referred as solid binding agentcontaining preferably polyethylene but also solid hydrogenated vegetableoils defined as C14-24 alkyl moiety triglycerides and derivatives.Powder metallurgical compositions are prepared by mixing at low shearconditions the binding agent with the metal-based powder and alloyingpowder at temperature just below the melting point of the binding agent.

Glyceryl esters of fatty acids are known as admixed lubricants for thecold compaction of iron-based powders. However, such esters do not yieldparticular remarkable densities when compared to powder compositionscomprising conventional lubricants, such as metallic stearates oramide-based waxes compacted under the same conditions.

SUMMARY OF THE INVENTION

The inventors have discovered that by compacting under specificconditions that iron-based powder compositions comprising specificglyceryl esters of fatty acids, powder compacts having unexpected highdensities can be produced while keeping good shaping behavior.Accordingly, the present invention provides a method for the fabricationof high density iron-based powder compacts, comprising compacting at atemperature range of 50 to 100° C., iron-based powder compositionscomprising a specific solid binder-lubricant having a melting pointbetween 50 and 100° C. and containing glyceryl esters of fatty acids.

The solid binder-lubricant could consist of a single glyceryl ester witha melting point falling in the range specified, or alternatively couldbe a mixture of different glyceryl esters. In the latter case, themelting point of the resulting mixture should lie in the specifiedrange, but it should be noted that the glyceryl esters forming thecomponents of the mixture could have individual melting points lyingoutside the specified range provided the resulting mixture has a meltingpoint falling within the range. Also, the binder lubricant may be mixedwith an additional lubricant, such as a polyolefin wax, which also has amelting point outside the specified range. The important point is thatthe binder-lubricant consisting of the glyceryl esters of fatty acids,whether it be a mixture or single component, has a melting point between50 and 100° C.

The invention also provides a metal powder composition comprising aniron-based metal powder and from about 0.01 to about 3 wt. % of aspecific binder-lubricant based on the total weight of the composition,preferably from about 0.05 wt. % to about 1.5 wt. %. The specificbinder-lubricant may be admixed to the metal powder in a solid state(comminuted, usually as a powder), in emulsion, in solution or in themelted state. The composition may further comprise other solidlubricants and/or binders and/or flowing agents to optimize either thecompressibility and lubrication performance, or the flow and thesegregation of the powder mixes. Alternatively, the specificbinder-lubricant can be mixed with another lubricant in the meltedstate, granulated to yield a powder and then admixed with the metalpowder composition. The specific binder-lubricant may also be sprayed onthe die walls of the tooling as a powder or as a solution.

Examples of iron-based powders are pure iron powders, powders of ironpre-alloyed with other elements, and powders of iron to which such otherelements have been diffusion-bonded. The composition may further containpowders of such alloying elements in the amount of up to 15 wt. % ofsaid composition. Examples of alloying elements include, but are notlimited to, elemental copper, nickel, molybdenum, manganese,phosphorous, metallurgical carbon (graphite) and ferro-alloys.

The glyceryl esters of fatty acids, also known as glycerides, can benatural or synthetic.

The metallurgical powder compositions of the invention can be compactedinto parts in a die and subsequently sintered according to standardpowder metallurgy techniques.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 shows the green density as a function of temperature;

FIG. 2 shows the In-Die density as a function of temperature;

FIG. 3 shows the axial springback after ejection as a function oftemperature;

FIG. 4 shows the In-Die density as a function of the compacting pressurefor a compacting temperature of 30° C.; and

FIG. 5 shows the In-Die density as a function of the compacting pressurefor a compacting temperature of 55° C.

DETAILED DESCRIPTION OF THE INVENTION

Several powder compositions were prepared and tested for the fabricationof ferrous compacts for P/M applications. Exemplary metal powderssuitable for the purpose of the present invention include iron-basedpowders used in the P/M industry, such as pure iron powders, pre-alloyediron powders (including steel powders) and diffusion-bonded iron-basedpowders. Substantially any iron-based powder having a maximum particlesize less than about 600 microns can be used in the composition of theinvention. Typical iron-based powders are iron and steel powdersincluding stainless steel and alloyed steel powders. ATOMET® steelpowders manufactured by Quebec Metal Powders Limited of Tracy, Quebec,Canada are representative of such iron and steel powders. TypicalATOMET® powders contain in excess of 99.6 wt. % iron and pre-alloyedmetals, less than 0.3 wt. % oxygen and less than 0.1 wt. % carbon, andhave an apparent density of 2.50 g/cm³ or higher, and a flow rate ofless than 30 seconds per 50 g.

Optionally, the iron-based powders can be admixed with alloying powdersin the amount of preferably less than 15 wt. %. Examples of alloyingpowders include, but are not limited to, elemental copper, nickel,molybdenum, manganese, phosphorus, metallurgical carbon (i.e. graphite)and alloys of the above, with or without iron.

Powder compositions of the invention include a specific binder-lubricantin an amount from about 0.01 wt % to about 3 wt % based on the totalweight of the composition, preferably from about 0.05 wt. % to about 1.5wt. %. This specific binder-lubricant may be admixed to the metal powderin a solid state (comminuted, usually as a powder), in emulsion or insolution. It could also be admixed to the metal powder in a solid state)and subsequently melted to bind the secondary powders to the basic metalpowder. The admixture may be carried out in a single operation or step,or in several steps. The composition may further comprise other solidlubricants or binders or flow agents to further improve either thecompressibility and lubrication performance, the flow and/or thesegregation of the powder mixes.

Typically, the specific binder-lubricant of the invention has a meltingpoint between 50 and 100° C., and is a mixture of natural or syntheticglyceryl esters of fatty acids also known as glycerides. Theseglycerides are typically mono-esters, di-esters or tri-esters ofglycerol (also known as monoglycerides, diglycerides or triglycerides),or a mixture of them. The C4-40 alkyl moiety of the glycerides can besaturated or unsaturated, linear or branched, and substituted orunsubstituted.

The binder lubricant of the invention may be used in combination with atleast one additional lubricant, which is preferably selected from thegroup consisting of non-metallic fatty acid compounds, such as ethylenebis-stearamide, stearic acid, oleic acid, and/or polyolefinic waxes,such as polyethylene wax. In this case, the preferred embodiment is abinder-lubricant composition comprising 5-95 wt. % or preferably 30-95wt. % of the binder lubricant of the invention and 95-5 wt. % orpreferably 70-5 wt. % of additional lubricant.

In some embodiments, the binder-lubricant of the invention is soluble instandard solvents, which makes it possible to prepare the binder-treatedpowder composition using spray coating techniques, or by other knowntechniques.

The metallurgical powder compositions of the invention can be compactedunder conventional powder metallurgy conditions. The compactingpressures are typically lower than 85 tsi and more specifically between10 and 60 tsi. The metal powder compositions of the invention can becompacted into parts in a die and subsequently sintered according tostandard powder metallurgy techniques. The compacting temperaturesuitable with the compositions of the invention is between 50 and 100°C.

PRACTICAL EXAMPLE

The compaction and ejection characteristics of the ferrous powdercompositions mixed using either dry, wet or melt-bonding procedures,were evaluated with a single action instrumented compacting device,known as the Powder Testing Center Model PTC 03DT, manufactured by KZKPowder Technologies Corporation, Cleveland, Ohio. This instrumentedpress allows continuous recording of the moving punch displacement, theforces applied to the moving punch and transmitted to the stationarypunch and the IN-die density all along the compaction and ejectionprocesses. The strippin g pressure, which corresponds to the forceneeded to start the ejection process divided by the friction area(contact surface between the compact and the die wall) and the ejectionunit energy were estimated from the ejection curve in order to comparethe lubricating performance of lubricants. The ejection unit energy isevaluated from the calculation of the area under the ejection curve(force vs. displacement) divided by the displacement of 2.54 mm and thefriction area.

Tests were also conducted on the wet and melt-bonded mixes to evaluatethe binding efficiency of the binder-lubricant of the invention. Dustingresistances were determined by fluidization with a stream of gas (inthis case air). Air was directed at a constant flow rate of 6.0liters/minute for ten minutes at the bottom of a 2.5 cm diameter tube inwhich the test material was placed. This causes finer secondary powders,such as graphite, to be entrained, as a result of a largesurface-to-volume ratio, and low specific gravity (in the case ofgraphite), and to be deposited in the dust collector. The mixtureremaining on the screen plate was then analyzed to determine therelative amount of alloying additive, which is a measure of theresistance to dusting when expressed as a percentage of the pre-testconcentration.

The typical binder-lubricant of the invention that was used in thefollowing examples is a mixture of mono, di and triglyceryl esters offatty acids, and is referred as GEFA. The mono, di and tri-estercontents are respectively: 8 to 22 wt %, 40 to 60 wt % and 25 to 35 wt%. The fatty acids entering in the composition of these esters arepalmitic acid (C16) and stearic acid (C18) in proportion of respectively40 to 60 wt % and 40 to 60 wt %. This binder-lubricant is in a powderform having an average mean diameter of ˜30-40 μm, and has a meltingpoint of 62° C.

EXAMPLE 1 Compressibility and Shaping

Two ferrous powder compositions containing 96.65 wt. % ATOMET 1001 steelpowder (Quebec Metal Powders Ltd.), 0.5 wt. % graphite powder (SouthWestern 1651), 2 wt. % copper powder (MD 165) and 0.75 wt. % oflubricant were prepared by conventional dry-mixing in a V type mixer.The first powder composition referred as Control mix contained atomizedACRAWAX C powder from Lonza Inc. (EBS) as lubricant, while the secondpowder compositions contained the binder-lubricant of the inventiondescribed previously (GEFA).

FIG. 1 illustrates the unexpected improvement of green density obtainedwhen compacting the powder composition comprising the binder-lubricantof the invention at temperatures higher than 50° C., as compared to theControl Mix. It can be seen that, at 25° C. similar green densities wereobtained for both powder compositions, while improved green densities ashigh as 0.15 g/cc at 55° C. and 0.2 g/cc at 65° C. were obtained for thepowder composition containing the binder-lubricant of the invention.

As illustrated in FIG. 2 and FIG. 3, these results can be explained inpart by the higher intrinsic compressibility, as shown by the in-diedensities results, and the lower axial springback after ejection, whencompacting temperatures of 55° C. and 65° C. and the powder compositioncontaining the binder-lubricant of the invention are used.

When comparing FIGS. 3 and 4, it can be seen also that the improvementof densities by compacting at 55° C., as compared to 30° C., the powdercomposition comprising the binder-lubricant of the invention, issignificant whatever the compacting pressure and even more when thecompacting pressure increases.

EXAMPLE 3 Separation Properties

Tests were conducted to evaluate the binding efficiency of thebinder-lubricant of the invention using either a melt-bonding procedureor a wet procedure.

Melt-bonding procedure: The binder-treated Mix 1 was prepared by mixingat a temperature of 65° C. close to the melting point of the GEFA typebinder-lubricant, 0.65 wt. % of GEFA with 96.5 wt. % ATOMET 1001 steelpowder (Quebec Metal Powders Ltd.), 0.85 wt. % graphite powder (SouthWestern 1651), 2 wt. % nickel (Nickel T123 PM, INCO Ltd). In this mix,the binder-lubricant of the invention was used both as binder andlubricant. The dusting resistance of the binder-treated Mix 1 powdercomposition was compared with the behavior of a dry mixture, referred asControl Mix 1 consisting in 95.5 wt % ATOMET 1001 steel powder (QuebecMetal Powders Ltd.), 0.85 wt. % graphite powder (South Western 1651), 2wt. % nickel (Nickel T123 PM, INCO Ltd) and 0.75 wt. % of atomizedACRAWAX C powder from Lonza Inc. (EBS).

Wet procedure: The binder-treated Mix 2 was prepared by dissolving 0.15wt. % of GEFA in a solvent and by mixing this solution with a mixture of96.43 wt. % ATOMET 4201 steel powder (Quebec Metal Powders Ltd.) and0.92 wt. % graphite powder (South Western 1651) and 2 wt. % copperpowder (SCM 50ORL) and 0.15 wt % of molybdenum (Sylvania) and 0.5 wt. %of zinc stearate (Pompla). This mixture was then dried by evaporatingthe solvent. The dusting resistance of the binder-treated Mix 2 powdercomposition was compared with the behavior of a dry mixture, referred asControl Mix 2 consisting in 96.93 wt % ATOMET 4201 steel powder (QuebecMetal Powders Ltd.), 0.92 wt. % graphite powder (South Western 1651), 2wt. % copper powder (SCM 500RL) and 0.15 wt % of molybdenum (Sylvania)and 0.5 wt. % of zinc stearate (Pompla).

As shown in Tables 1 and 2 below, the graphite, nickel, copper andmolybdenum dusting resistances provided by the Binder-treated Mixes aresignificantly improved as compared to the Control Mixes. TABLE 1 Dustingresistance of the Control Mix 1 and Binder-treated Mix 1 Wt. % Graphitedusting Nickel dusting Control Mix 1 57 39 Binder-treated Mix 1 88 61

TABLE 2 Dusting resistance of the control Mix 2 and binder-treated mix 2Graphite Molybdenum dusting Copper dusting dusting Wt. % resistanceresistance resistance Control Mix 2 33 16.7 47 Binder-treated Mix 2 9146 79

1. A method of producing a compacted powder composition, comprising: a) adding about 0.01 wt. % to about 3 wt. %, based on the total weight of the composition of a binder-lubricant to an iron-based powder, said binder-lubricant consisting essentially of one or more glyceryl esters of fatty acids and having a melting point between 50 and 100° C. and; b) compacting the dry metallurgical powder composition at a temperature in the range of 50 to 100° C.
 2. The method of claim 1, wherein from about 0.05 wt. % to about 1.5 wt. % of the binder-lubricant is added in step a).
 3. The method of claim 1, wherein the iron-based powder contains up to about 15 wt. % of one or more alloying elements.
 4. The method of claim 4, wherein the alloying elements are selected from the group consisting of: elemental copper, nickel, molybdenum, manganese, phosphorus, and metallurgical carbon.
 5. The method-of claim 1 further comprising adding an additional lubricant to the powder metallurgical composition prior to compacting in step b).
 6. The method of claim 5, wherein the additional lubricant is a non-metallic fatty acid compound, polyolefinic waxes, or combination thereof
 7. The method of claim 6, wherein non-metallic fatty acid compound is selected from the group consisting of: ethylene bis-stearamide, stearic acid, oleic acid.
 8. The method of claim 6, comprising 5-95 wt. % of binder-lubricant and 95-5 wt. % of additional lubricant.
 9. The method of claim 6,.comprising 30-95 wt. % of binder-lubricant and 70-5 wt. % of additional lubricant.
 10. The method of claim 1 further comprising adding another binder to the powder metallurgical composition prior to compacting in step b).
 11. The composition of claim 1, wherein the glyceryl esters of fatty acids are selected from the group consisting of: mono-esters, di-esters, tri-esters of glycerol, and mixtures thereof.
 12. The method of claim 1, wherein the binder-lubricant comprises glyceryl esters of fatty acids having a C4-40 alkyl moiety that can be saturated or unsaturated, linear or branched, and substituted or unsubstituted.
 13. The method of claim 1 wherein the iron-based powder has a maximum particle size less than about 600 microns.
 14. A method of producing a metal component, the method comprising sintering the compacted powder composition of claim
 1. 15. A compacted powder composition, comprising about 0.01 wt. % to about 3 wt. %, based on the total weight of the composition of a binder-lubricant to an iron-based powder, said binder-lubricant consisting essentially of one or more glyceryl esters of fatty acids and having a melting point between 50 and 100° C.
 16. The compacted powder composition of claim 15, comprising from about 0.05 wt. % to about 1.5 wt. % of the binder-lubricant.
 17. The compacted powder composition of claim 15, wherein the iron-based powder contains up to about 15 wt. % of one or more alloying elements.
 18. The compacted powder composition of claim 17, wherein the alloying elements are selected from the group consisting of: elemental copper, nickel, molybdenum, manganese, phosphorus, and metallurgical carbon.
 19. The compacted powder composition of claim 15, further comprising an additional lubricant.
 20. The compacted powder composition of claim 19, wherein the additional lubricant is non-metallic fatty acid compound, polyolefinic waxes, or combination thereof.
 21. The compacted powder composition of claim 20, wherein the non-metallic fatty acid compound is selected from the group consisting-of: ethylene bis-stearamide, stearic acid, oleic acid.
 22. The compacted powder composition of claim 19, comprising 5 -95 wt. % of the binder-lubricant and 95-5 wt. % of additional lubricant.
 23. The compacted powder composition of claim 19, comprising 30-95 wt. % of the binder-lubricant and 70-5 wt. % of additional lubricant.
 24. The compacted powder composition of claim 15 further comprising an additional binder.
 25. The compacted powder composition of claim 15, wherein the binder-lubricant comprises glyceryl esters of fatty acids having a C4-40 alkyl moiety that can be saturated or unsaturated, linear or branched, and substituted or unsubstituted.
 26. The compacted powder composition of claim 15, wherein the iron-based powder has a maximum particle size less than about 600 microns. 